Ink jet printing is a non-impact method that produces droplets of ink that are deposited on a substrate such as paper or transparent film in response to an electronic digital signal. Thermal or bubble jet drop-on-demand ink jet printers have found broad application as output for personal computers in the office and the home.
Aqueous inks used in ink jet printing have water as a major component. Water has the advantage of being non-toxic, non-combustible and environmentally sound relative to non-aqueous inks which are largely composed of organic solvents. Water is also an excellent media for dispersed pigments or dissolved dyes.
The use of water in large concentrations, however, also has several disadvantages. Water has a slow evaporation rate relative to low-boiling organic solvents, which reduces the rate of drying and, thus, the rate of printing. Water also interacts with paper to cause two major distortions known as paper cockle and paper curl. Paper cockle is a distortion in which bumps, indentations and other irregularities are randomly produced on the printed paper, giving the paper a "wrinkled" appearance. Curl is a phenomena in which the edges of the paper migrate towards the center of the paper. The direction of curl may be on the printed side of the paper, or it may be on the non-printed side (the latter being known as "reverse curl").
Curl may appear immediately after printing or may take a day or two to manifest. In its final state, the paper sheet may take the form of a tube. Curled paper cannot be stacked, sheet upon sheet, thereby causing much inconvenience to the user. Curled sheets are difficult to display or store and cannot be used in processes requiring near planarity, such as media feeding, tracking, and print alignment. Curl is most prevalent in solid fill printing and is therefore a more acute problem in graphics as opposed to text printing. For the same reason, it is mostly a concern in 4 color printing where graphics are prominent. The use of heating elements (commonly employed to increase the rate of drying of aqueous inks) are known to accelerate paper curl.
Various mechanical devices to reduce curl such as heated rollers and tension applicators have been tried. These devices are only marginally effective and add considerably to the cost and size of the printer. (Heated rollers used to reduce curl differ from the heaters used to increase drying rate; in the former heat is applied to both sides of the paper after printing whereas in the latter heat is applied during the printing process.)
It is also known to reduce curl by modifying the print media. This approach is highly undesirable, however, because it is expensive and because consumers have an overwhelming preference for printers which can use a wide variety of office papers, especially those sold into the electrophotographic copier paper market, or so-called "plain-paper" printers.
There is thus a need in the art for aqueous ink compositions which can be printed in full page graphics without producing paper curl, thus eliminating the need for expensive, ineffective and cumbersome mechanical devices or special print media.
The ink compositions of the present invention fulfil the above need without any deleterious effects such as inducing pluggage, kogation, inferior print quality, or instability in storage, and at much lower cost than can be obtained by equipment or media modifications.